System for converting light energy into electrical energy and vice versa



F. MOHR June 24, 1930.

SYSTEM FOR CONVERTING LIGHT ENERGY INTO ELECTRICAL ENERGY AND VICE VERSA Original Filed April 6, 1927 20/54 avg a 0 comcom Patented June 24, 1930 UNITED- s'rA-TEs PATENT. OFFICE rmnm norm, or'nEw YORK, Y., nssrenon' ro BELL TELEPHONE nanoiaa'ro- Ems, INCORPORATED, or NEW YORK, N. Y., a conronn'rrou on NEW Yon];

sxsrm Ion comma LIGHT ENERGY m'ro ELECTRICAL ENERGY m v1cE Application filed April 6, 1927, Serial m. 181,533. Renewed m 25, 1929.

This invention relates to distortion correction in systems employing scanning" operations andmore particularly in television, telephotographic and sound recording and re- 5 producing systems.

An object of the invention is to improve the operation of systems employing scanning by taking account of, and compensatin for, at least in part, the distortion due to't e finite dimensions of.scan ned elemental areas.

Considering for example, ordinary picture transmission systems and television systems in which the light from successive small areas of an optical field is used to set up current variations in an electrical circuit, and these variations are used to control a beam of light at the receiving station to produce the image, the elemental areas of the field. at the transmitter and those at the receiver which are at any instant illuminated and scanned are necessarily of finite size. The result of the finite dimensions of the elemental areas involved in the scanning operation is to produce distor tion in that neither the electrical waves produced by the light at the transmitting station nor the impression upon the photographic film or the eye produced by the light at the receiving station exactlycorresponds respectively to the point-to-point variations of the light tone values of the field which is scanned at the transmitter and the current which is received at. the receiver. Similarly, every scanning operation involving elemental areas of finite dimensions however performed introduces distortion. due to the finite dimension of the elemental areas in the direction of scanning.

Applicant has discovered that distortionof this type can be represented as an attenuation variable with respect to frequency and that, for a fixed width of aperture and velocity of scanning (that is, velocity of eitherthe image or aperture with respect to the other) it can be corrected, at least in part, by means of an attenuation-equalizer of fixed value in the electrical circuit.

In a specific embodiment herein" described for purposes of illustration, this invention comprises a picture transmission system employing a rotating disc with a serles of aperimage to be transmitted is scanned in line tures of fixed size by means of which the series of elemental-areas of finite dimensions as the disc rotates. The light passing through the apertures impinges on a photoelectric cell generating electrical waves, which represent the integrated light of each elemental area of the-image exposed.

electrical network having such an attenuation characteristic as to tend to correct for the distortion produced by the finite dimensions of the elemental areas is connected in the electrical circuit so that the final electrical wave corresponds substantially to the time variations of the tone value of the elemental areas U of the picture as it is scanned. Similarly, at

the receiving end of the system there is employed an electricalnetwork whichwill so attenuate the electrical waves as to at leeast largely compensate for the distortion produced by the finite dimensions of the elemental areas'in the direction of the apertures of the receiving disc, with the result that the final exposurewill more nearly correspond to the original image. Obviously, if desired, the system may be modified by employing one electrical network for'correcting for the distortions produced by the finite dimensions of the elemental areas in the direction of motion at both the transmitting and receiving sta tions.

Although this invention is illustrated in connection with a picture transmission system, it is also applicable to other systems employlng apertures of fixed size or equivalents, as hereinafter explained in more detail.

- ,the ordinates indicating on a percentage basis the,current transmitted by the system at any particular frequency without the aid of equalizing networks; Fig. 3 is also a scanning response curve based on the same data as the curve shown in Fig. 2 but showing the relation of frequency to loss in transmission units.

and in miles; and Fig. 4 is a tone value-dis- An aperture of finite dimensions when positioned opposite any part of a picture or scene to be transmitted will usually cover an area which is not uniformly illuminated. One edge of the exposed area may, for example,

be very dark, while the other edge may be very bright. The pencil of light passing through the aperture from this area to'the photo-electric cell will give rise to a current which will be proportional to the average brightness of the area. As the aperture and picture move with respect to each other, the current varies in accordance with the average brightness of the picture over its elemental areas. If the motion carries the aperture over a sharp vertical division between white and black, the resulting current will vary from a maximum before the aperture reaches this division line, to a minimum after the aperture leaves the division line, through a constantly diminishing value as more of the aperture is covered by the black. The opposite 'efiect occurs when the aperture moves over a sharp vertical division between black and white. If now the picture is made up of a number of vertical stripes of black on a white background, and the stripes are narrower than the aperture, even with the whole stripe centered behind the aperture some white will be exposed and the photoelectric current will never reach the minimum corre sponding to'black. Such results occur when the details of the picture to be transmitted are comparable in size to the dimensions of the aperture. They produce, in the reproduced picture, a blurred effect where there should be an abrupt change in shade.

A mathematical expression for the distorting effect of the finite width of aperture may be developedand this will first be discussed.

Any finite elemental area may be considered as made up of a number of component areas each one small enough so that its tone value does not perceptibly vary. These component areas may be considered as being arranged in rows and columns, and a beam of light transmitted by a practical aperture may be considered as made up of numerous small beams, each produced by one of the component areas. As the aperture and film move past one another each component beam varies in intensity with the nature of the optical field. Each component beam contributes -a certain amount of light to the illumination of the photoelectric cell and the total light incident on the cell at any instant is the sum of thecontr'ibutions from all the component areas.

Assume now that the aperture is moving past the picture with a uniform speed in a direction parallel to the rows and perpendicular to the columns of component areas. Any one row of'component areas has the same succession of picture light intensities and constitutes a strip having the width of a single component area. The illumination received by the photoelectric cell from each component area in the row goes through the same series of variations, but will be in slightly diiferent phase for each such area.

A column of component beams may be considered as aunit contributing a total light flux corresponding to the average density in a narrow transverse band of the picture. The contribution of a column to the illumination of the photoelectric cell may be represented by a curve or wave showing the illumination as a function of time or of distance along the picture such as shown in Fig. 4. Successive columns in the'entire elemental area (where rectangular) contribute the same light variations,b1it at a later time so that their contributions may be represented b a series of curves like that of Fig. 4 in diflerent phase positions. The sum of the curves representing the contributions of all the columns gives a single curve which represents approximately the photoelectric current as a function of time. The exact form of the current curve may be found by assuming the component areas to be infinitesimally small and integrating to obtain the total effect. This process assumes, of course; that the electrical circuits associated with the photoelectric cell introduce no change in the wave form.

The fundamental curve corresponding to a single column of component areas may be resolved into single frequency components by means of Fouriers series, of Fouriers integral as the case requires. This fundamental wave represents the light variations of the-picture and also the current which would result from scanning the picture with a beam of unvarying dimensions produced by a single infinitely narrow aperture. The curve repre senting the actual current is therefore obtained by adding together a large number of copies of the fundamental wave displaced in phase by various amounts. Alternatively the have an amplitude A, phase angle zero and a wave length KX (X being the width of the aperture) The wave length will have this definite relation' to the aperture width because of the uniformity of the scanning velocity..

If x denotes the distance along the picture I I=A SID I where A is the distance of the elemental aperture from the centre of the ph sical aperture.

The electrical response dy no to li 'ht fiux through the elemental aperture of wldth dA will be proportional to t e product Ida, or

The response due to the Whole square aperture which is designated by y, is therefore and hence Equation (3) may be written which expresses the electrical response in terms of the time t, and a frequency defined by ince t= Ka 8 The amplitude of the response varies in accordance with the factor K Si1l%r (6) which, accordingly, is a measure of the distorting effect of the aperture. Since an aperture such as is used in practice is not infinitely narrow but has a finite width, there is a discriminatory response with respect to aperture width and the Wave lengths of the component variations in the pattern of the picture or field, as evidenced by the expression (6), in which the amplitude of the response is given as a function of the ratio of wave length to aperture-width.

The above analysis shows that the action of the aperture is to set up a current having frequency components of incorrect relative amplitudes. Compensation may be effected by introducing a complemental frequency discrimination. p

The derivation of a mathematical expression for a square-shaped aperture as given above illustrates the manner in which similar derivations may be made for apertures of other shapes; for example, circular and elliptical apertures. The derivation of an expression fora circular aperture and comparison of the equations for square and circular apertures is subsequently made herein.

Fig. 2 and Fig. 3 represent the variation with frequency of the electrical components transmitted by the system and are generally typical for different scanning velocities and for different widths of aperture, thoughthey are based on calculations for the particular case in which the scanning velocity is 8.5 inches per second, and the aperture width is 0.015 inches. The curves show that at certain frequencies the response falls to zero. These frequencies correspond to the cases where the aperture width is equal to one or more wave lengths of the component waves of the space-density distribution. In practice it is desirable to use an aperture of small enough width so that frequency of the first zero point is above the highest necessary for a good reproduction of the picture.

It is seen from the foregoing analysis of the action of a square aperture a scanning area of any shape may be resolved into a number of narrow elemental strips having lengths equal to the chords of the scanning aperture area and extending lengthwise in a direction parallel to the direction of the scanmng.

Each such elemental strip or chord constitutes an elemental scanning area having an infinitesimal width and a finite length, measured in the direction of the motion, equal to the length of the chord.

Except for a specially selected scanning area, for example,'one in the form of a square or parallelogram, the elemental scanning strips or chords comprising the complete scanning area will not be of equal length but the length of each will depend upon the location of the particular strip or chord.

The distorting efi'ect of any one of the component elemental scanning areas is given as before by Equation (5), the symbol K therein being given a different value for each length of elemental area.

The distorting effect of the complete scanning area is for all practical purposes a Weighted average of the distorting effects of the component scanning areas.

On the right hand side of Equation (5), the sole factor. which determines the distorting eflect due to the shape of the scanning area is sin This will be apparent when it is observed that KX is by definition the wave length of a component of the pattern of the optical field or image. The dimensions of the 'pattern'are, of course, unchanged by anyvariation in the'shape of the scanning area.

5 1 In thecase of a scanning area of circular outline, the average value of the factor sin 1% weighed for the varied lengthsof the component scanning areas of the aperture, may be derived by the following mathematical integration'.

Forpurposes of convenience in performing the integration, the symbols K and hwill be introduced but these symbols will be eliminated from the mathematical expressions before'the final result is reached. K denotes the particular value of K corresponding to a rectangular aperture of the'same length as the diameter of the circular aperture under consideration. The symbol h denotes the distance of any component elemental rectangular aperture or chord from the center of the v circular aperture. 26 The value of K for elemental aperture is dependent upon the distance h of the chord from the center of the 1 whole circular aperture, in a manner defined by the following equation:

K0 v v f T 2 z This value of K may be substituted in th as expression sin and the latter integrated between the limits h=i The average value of the integral by the. difference 1 between the limiting values of h. The result is given by the following equation:

. 0 To facilitate a transformation of the above definite integral into a standard form, the solution of which is given in mathematical treatises, it is necessary to make a substitution of a new variable 0,- defined as follows:

any particular chordal By reference to Formula I on page 21 of A Treatise on the Theory .of Bessel Functions, by G, N. Watson, University Press,

Cambridge, 1922, the'evaluation'of the definite integral (13) is seen to be given by the following equation:

In Equation (14) J1 signifies a Bessel function of the first order, for the argument- 1 0 wave length of a given component of the pattern of the optical field or image and the diameter of the circular scanning area;

- I The ratio K is measured as between the The diameter of a circularaperture of the same areaas a square aperture, with a side of 2 length X, is Since KX=)., for all values of )t, the value of K for the circular v 2 i aperture of d1ameterX 1s necessanly K. H. c i n 2 40 value of sin s obtained by d viding the The amplitude A was used in Equation (1) in connection with the response from the square aperture in order to proportion the response to the width of the aperture measured'at right angles to the direction of scanning. In calculating the response from a circular aperture of equal area to the square aperture'the amplitude A must be multiplied 2 by I I r q I The distorting effect of the circular aper ture of area equal to that of the square apersubstit uting 11 from the right hand side of Equation (14), forsin in Equa tion (5) and in addition SIIbStltUtlBg Y K for K, and for A for the reasons given above. The result of these-substitutions is AKX 2 2n m J,( )s1n t* (15)- v ture willbe denoted by y and is found by i shows that the right handmembers differ in 1 the numerical constant factors andXT -respectively, and in the functional expressions sin and J trigonometric and Bessel functions, respectively.

The responses of the two. apertures if of equal area must, however, come to be identical in case each is used to scan a uniform field of the same intensity. The value of K for such a field with any finite aperture is infinite, because the wave length of the pattern of the field is then infinite in comparison with the length of the aperture. For K infinite, the limiting values of the respective functions are as follows:

II 11 p 81112 Under these limiting conditions 2H =AXsm t (18) Thus it is evident that when K= oo yam/ As is well understood, light which is employed for scanning may be either transmitted directly from the source or may be refracted or reflected before reaching the object or field being scanned. If small mlrrors were distributed around a scanning disc in the usual position of the holes therethrough, reflected instead of transmitted light would be used. The boundary of each mirror would then determine an optical aperture. The term aperture is herein used to denote such an optical or light aperture. The shape and size of the aperture determines the shape and size of the boundaries of the light beam, other conditions being fixed. The light beam may, of course, consist either of'parallel, convergent or divergent rays.

The term light is used herein in its broader sense to include not only wave lengths within, but also wave lengths both above and below, the visible spectrum. The term optical is likewise used in its broader sense. The field, as the term field is employed herein, may be at a transmitting point or a receiving point. It may be either primary or secondary-that is, it may comprise physical ohjects or it may merely contain images of such objects, and the images may be either real or virtual.

In many picture transmission systems, th

electric current is used at the receiving end to vary the intensity of the light which passes through a fixed aperture and illuminates an elemental area of fixed size, the illumination being uniform over the entire aperture and over the entire elemental area at any instant. As in the case of the aperture at the transmitting end, the aperture at the receiving end may be conceived of as being. divided into partial apertures, and a column of such apertures may be considered as a unit. such single column exposes a strip of film so that its tone values will follow the variations of the electric current and each column making up the total aperture will produce the same series of tone values, but the series will be displaced by varying distances along the surface of the received picture. Assuming that the film or other receiving surface does not introduce additional distortion, the exposures made by the various columns of partial apertures may be added together and are equally effective in producing the final tone values. As a consequence, the frequency characteristic of the receiving aperture can be calculated in the manner above described for the transmitting aperture.

The following examples will serve to illustrate ways in which the receiving aperture distorts the received picture. If the current wave suddenly changes from one steady value to another the amount of light going through the aperture changes accordingly but the vreceived picture shows a gradual change extending over the distance covered by the entire aperture at the instant of the change in current. Again, if the current has a component at one of the critical frequencies that component will be absent entirely in the received picture. i

Knowing the frequency characteristic or the equationsof electrical response, Equations (5)- and (15), of the apertures, and the size of the apertures and the rate of scanning, an electrical equalizer may be designed from the well known principles of such design bearing in mind the limitation that at the frequencies at which the aperture width equals one or more complete wave lengths there is a complete loss.

A pictu e transmission system including equalizers having suitable characteristics for use over theessential range of frequencies is s own in Fig. 1. The picture is represented by film 1 illuminated by lamp 2.. This film may be a moving picture film. An image of the picture is focused on a disc 3 in which 'the' scanning apertures are arranged as shown in Fig. 1. The light passing through the apertures impinges on photoelectric cell 4 generating a. photoelectric current. Modulator 5 causes this photoelectric current to modulate the carrier current generated by source 6. This modulated wave is then amplified, by amplifier 7, passed through equalizer 8 and filter'9 to the transmission line 10. Rectangles 11 and 12 indicate the apparatus employed for maintaining the d1sc 3 and the receiving scanning disc 18 1n synchromsm. The apertures in disc 18 are arranged in the same Way as those in disc 3.

At the receiving station the filter 13 separates the synchronizing current from the picture current; equalizer 14 attenuates certain frequencies of the received current to comensate for the distortion to be introduced Ey the aperture of disc 18; amplifier 15 increases the strength of the current and demodulator 16 separates the variations representing the photoelectric current from the carrier war e. The demodulated currentthen the recording means 20 registers a moving beam of light which varies in intensity mak claimed in Patent No. 1,606,817, granted to G. H. Stevenson, November 16, 1926 though obviously other types well known in the art may be employed.

While the invention is illustrated as embodied in either a still or moving picture transmission system, it isv equally applicable to television systems or other signal systems employing apertures or'their equivalents for scanning elemental areas of finite dimensions of a' field, either in transmission or in reception or in both. In television transmission the apparatus at the transmitting and at the receiving stations is arranged to transmit and produce, respectively, images within the period of the persistence of vision. This invention may be applied to other systems such as a system for the photographic recording and reproducing of sound. In such a system the incoming electrical waves representing the sound are impressed upon the sound recording apparatus at the receiving station so as to affect at each instant a finite area of a film or other recording element arranged to move continuously past a given point. In suchan arrangement light may be transmitted from a sound controlled receiving light source through a fixed aperture to a moving The sound record may thus be made on the film with somewhat simpler apparatus than required for picture transmission, since the sound record canbe made in a continuous line upon the recording film. When this sound record is used for revaries the intensity of the light emitted byy id aperture.

lamp 117. As disc 18 moves past the larnp 17,;

1. In a system for converting light energy into electrical energy or vice versa, a light source, a control element for light from said source having an aperture associated therewith, an optical field, means for causing relative motion between said aperture and said field, and means having such a frequencyattenuation characteristic as to compensate,

in part at least, for the distortion produced 2." In a system for converting light energy into electrical energy or vice versa, a light source, a control element for light from said source having an aperture of fixed size associated therewith, means for producing a relative motion between said aperture and the optical field, and an electrical network having a frequency-attenuation characteristic dependent upon the size of said aperture and the velocity of the motion of the aperture and optical field relative to one another, to compensate, in part at least, for the distortion produced by said aperture.

3. In a system for converting light energy into electrical energy, a device for producing electrical waves under control of the light energy from an optical field, a light aperture of fixed size associated therewith, means for producing a relative motion between the aperture and the optical field, and an electrical network having a frequency-attenuation characteristic dependent upon the size of the aperture and the velocity of motion of the aperture with respect to the optical field for compensating, in part at least, for the distortion produced by said aperture.

4. In a system for converting electrical waves into light energy, a source of electrical waves, a device producing light energy corresponding to electrical waves, a surface to be affected by the light energy, an aperture of fixed size, means for producing a relative motion between the aperture and the surface, andan electrical network having a frequency attenuation characteristic dependent upon the size of the aperture and the velocity of motion of the aperture relative to the surface for compensatingfin large art at least, for

the distortion due to the nite size of the aperture in the direction of said motion.

5. In an electro-optical system, means for producing variations corresponding to the successive, separate, integrated tone values of successive, finite, overlapping elemental areas successively infinitesimally displaced along a path on a picture or other object whose I image is transmitted, and frequency discriminating means for so modifying said variations as to produce variations corresponding more nearly to the successive tone values of the successive infinitesimal portions of said path which constitute said displacements.

6. In an electro-optical system, means for producing electrical variations corresponding to the successive, separate, integrated tone values of successive, finite, overlapping elemental areas successively infinitesimally displaced along a path on a picture or other object whose image is transmitted, and an electrical network so constructed and proportioned and associated with said means as to produce variations. corresponding more nearly than said first variations to the successive tone values of the successive infinitesimal portions of said path which constitute said displacements.

7. The method of compensating in a transmission system for the distortion efiects produced by a moving aperture of finite area which comprises so attenuating electrical waves, produced in response to light waves passing through said aperture, as to compensate, in large degree at least, forthe distortion produced by said aperture due to its finite dimension in the direction of motion.

8. In a transmission system in which electrical waves are produced in response to light Waves passing through an aperture, the method of compensating for the distortion in the photoelectric response characteristic resulting from a definite ratio of aperture width to speed of scanning, which comprises so attenuating the electrical waves, as to compensate, in large degree at least, for the distortion.

9. In a system in which light Waves are produced in response to electrical waves and transmitted through an aperture to a light sensitive surface, the method of compensating for the distortion effects produced by the aperture which comprises so attenuating the electrical waves that the exposure of the light waves substantially corresponds to the time variations in the electrical waves.

10. The method of electro-optical transmission which comprises scanning successive elements of a path on a picture surface, producing electrical variations corresponding to the tone values of the finite elemental portions of the path as they are successively traversed in said scanning, and attenuating said electrical variations by such differentamounts for difierent frequencies as to compensate in the direction of the scanning, in large degree at least, for distortion introduced in the picture transmission by said scanning. I

11. The method of electro-optical transmission, which comprises producing sets of variations, each set in different time phase and representing the density variations along the same path on a picture surface, combining at each instant said sets of variations to produce a resultant set of variations, producing an electrical characteristic varying in a complex manner corresponding to said resultant set of variations, and transmitting the different frequency components of the varying electrical characteristic with such relatively dilferent attenuations as to compensate, in part, for distortion introduced in said transmission due to the fact that the value of the electrical characteristic at each instant represents a portion of the picture having a plurality of tone values.

12. In electro-optical transmission, the

method which comprises producing a light beam moving relatively to a picture surfaceand having a finite dimension in the direction of said motion and having illuminating power varying in accordance with the tone values of the elemental picture areas successively traversed, producing electrical variations corresponding to said variations of illuminating power, and transmitting different frequency components of said electrical variations with such difierent relative transmission efficiencies as to compensate, in part, for distortion introduced in said transmission due to the finiteness of said dimension.

13. In a system for converting light energy into electrical energy or vice versa, means for scanning a field comprising means'for projecting a beam of light, means for producing relative movement between said beam and said field, and frequency discriminating means for compensating, in large degree at least, for distortion due to the finite dimension of said beam in the direction of relative movement.

14;. The method of compensating in a trans; mission system for the distortion effects produced by an aperture of finite width which comprises so attenuating electrical waves having characteristics corresponding to light waves passing through said aperture as to compensate, in large degree at least, for the distortion produced by said aperture.

15. The combination of means for scanning in succession small areas of a surface or field and frequency discriminating meansfor" compensating, in large part at least, for'dis tortion n uced by the scanning operation sate for distortion due to the finite width in the direction of scanning of said elemental areas.

17 In an image producing system in which image impulses characteristic of light tone values of the object or field of View at the transmitter are utilized, the combination of nieans for scanning in succession small areas of a field, the width of said areas in the direction of scanning being not greater than the shortest Wave length of the object or field structure which it is desired to reproduce in the image, and frequency discriminating means through which the image impulses pass for substantially compensating for distortion due to the finite width of said elemental areas in the direction of scanning. 18. The method of compensating in atransmission system for the distortion effects produced by a moving aperture of finite area which comprises so attenuating electrical waves, produced under control of said aperture as to compensate, in large degree at least, for the distortion produced by said aperture due to its finite dimension in the direction of motion. 7

In witness whereof, I hereunto subscribe my name this 6th day of April, A. D. 1927.

FRANIUJIN MOHR. 

